The present invention relates to devices for controlling tilt of forklift masts.
A typical forklift includes a mast and a fork. The mast is supported by a vehicle body so that the mast tilts. The fork is supported by the mast so that the fork is lifted or lowered. The forklift also includes a tilt cylinder and a control valve. The tilt cylinder enables the mast to tilt forward or rearward with respect to the vehicle body. The control valve controls an oil supply for the tilt cylinder. A tilt lever is arranged in the vicinity of the operator seat of the forklift. By shifting the tilt lever, the opening of the control valve is varied so that the tilt cylinder operates to tilt the mast.
The opening of the control valve varies in correspondence with the position of the tilt lever, or the angle of the tilt lever. The flow of oil supplied to the tilt cylinder varies in correspondence with the opening of the valve. Such flow determines the tilt speed of the mast.
The mast is supported by the vehicle body at the lower end of the mast. Thus, regardless of the tilt speed of the mast, the tilt speed of the fork is greater when the position of the fork is higher. If the tilt lever is shifted rapidly to its maximum tilt angle, the mast starts to move immediately and tilts at a high speed. In this case, if the fork is located at a high position are carries an object, the object may become unstable or fall from the fork. It is also possible that a rear wheel of the forklift may be raised from the ground. It is thus necessary to move the tilt lever carefully when the fork is located at a higher position.
To solve the above problem, Japanese Unexamined Patent Publication No. 5-229792 describes a device for controlling the tilt speed of the mast in correspondence with the height of the fork. This device includes sensors for detecting the height of the fork, the weight of the object carried on the fork, and the position of the tilt lever. A controller controls opening of a proportional electromagnetic type control valve in accordance with detection values of the sensors, thus varying the flow of the oil supplied to the tilt cylinder. Specifically, the controller varies instruction values for the opening of the control valve in correspondence with the height of the fork) the weight of the object and the position of the tilt lever. Thus, if the fork is located at a higher position and carries an object, the mast is controlled to tilt at a lower speed. In this manner, even when the tilt lever is shifted to the maximum speed position, problems related to instability do not occur.
The above described control valve includes a solenoid that operates to vary the opening of such valve. While detecting the position of the tilt lever, a controller varies the value of the current supplied to the solenoid as an instruction value in correspondence with variation of the lever position. However, this control method results in a time lag between the shifting of the tilt lever and the operation of the tilt cylinder in response to the position of the tilt lever. That is, the operation of the tilt cylinder does not respond quickly to the shifting of the tilt lever, and the manipulation of the tilt lever is thus difficult.
Accordingly, it is an objective of the present invention to provide a tilt control device for a forklift that restricts the maximum tilt speed of a mast when a fork is located at a higher position and facilitates manipulation of a tilt lever for tilting the mast.
To achieve the above described objective, a tilt control device of a forklift mast according to the present invention includes a hydraulic cylinder for tilting the mast. A first valve is provided for controlling supply of a fluid to the cylinder for cylinder operation. The device also includes an operating member for operating the first valve. The first valve supplies fluid to the cylinder in correspondence with the position of the operating member. The cylinder tilts the mast at a speed corresponding to the flow rate of the fluid supplied by the first valve. A fluid passage is arranged between the cylinder and the first valve. A second valve is provided for restricting the maximum flow rate of the fluid passing through the fluid passage. The second valve varies the maximum flow rate depending on the position of the fork.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.